DYNAMIC CHARACTERISTICS OF THYRISTOR

Thyristor Turn-ON Characteristics

Fig.3.7: Turn-on characteristics

When the SCR is turned on with the application of the gate signal, the SCR does not conduct fully at the instant of application of the gate trigger pulse. In the beginning, there is no appreciable increase in the SCR anode current, which is because, only a small portion of the

silicon pellet in the immediate vicinity of the gate electrode starts conducting. The duration between 90% of the peak gate trigger pulse and the instant the forward voltage has fallen to 90% of its initial value is called the gate controlled / trigger delay time gdt . It is also defined

as the duration between 90% of the gate trigger pulse and the instant at which the anode current rises to 10% of its peak value. gdt is usually in the range of 1 sec.

Once gdt has lapsed, the current starts rising towards the peak value. The period during which

the anode current rises from 10% to 90% of its peak value is called the rise time. It is also defined as the time for which the anode voltage falls from 90% to 10% of its peak value. The summation of gdt and rt gives the turn on time ont of the thyristor.

Thyristor Turn OFF Characteristics

When an SCR is turned on by the gate signal, the gate loses control over the device and the device can be brought back to the blocking state only by reducing the forward current to a level below that of the holding current. In AC circuits, however, the current goes through a natural zero value and the device will automatically switch off. But in DC circuits, where no neutral zero value of current exists, the forward current is reduced by applying a reverse voltage across anode and cathode and thus forcing the current through the SCR to zero.

Anode current

begins todecrease

tCtq

t

t

Commutationdidt

Recovery Recombination

t1 t2 t3 t4 t5

trr tgr

tq

tc

VAK

IA

tq=device off time

tc=circuit off time

As in the case of diodes, the SCR has a reverse recovery time rrt which is due to charge storage in the junctions of the SCR. These excess carriers take some time for recombination resulting in the gate recovery time or reverse recombination time grt . Thus, the turn-off time

qt is the sum of the durations for which reverse recovery current flows after the application of

reverse voltage and the time required for the recombination of all excess carriers present. At the end of the turn off time, a depletion layer develops across and the junction can now withstand the forward voltage. The turn off time is dependent on the anode current, the magnitude of reverse gV applied ad the magnitude and rate of application of the forward

voltage. The turn off time for converte grade SCR’s is 50 to 100 sec and that for inverter grade SCR’s is 10 to 20 sec.

qt

To ensure that SCR has successfully turned off , it is required that the circuit off time ct be

greater than SCR turn off time qt .

Thyristor Turn ON

Thermal Turn on: If the temperature of the thyristor is high, there will be an increase in charge carriers which would increase the leakage current. This would cause an increase in 1 & 2 and the thyristor may turn on. This type of turn on many cause thermal run away and is usually avoided.

Light: If light be allowed to fall on the junctions of a thyristor, charge carrier concentration would increase which may turn on the SCR.

LASCR: Light activated SCRs are turned on by allowing light to strike the silicon wafer.

High Voltage Triggering: This is triggering without application of gate voltage with only application of a large voltage across the anode-cathode such that it is greater than the forward breakdown voltage BOV . This type of turn on is destructive and should be avoided.

Gate Triggering: Gate triggering is the method practically employed to turn-on the thyristor. Gate triggering will be discussed in detail later.

dvdt

Triggering: Under transient conditions, the capacitances of the p-n junction will

influence the characteristics of a thyristor. If the thyristor is in the blocking state, a rapidly rising voltage applied across the device would cause a high current to flow through the device resulting in turn-on. If

2ji is the current throught the junction 2j and

2jC is the junction capacitance and

2jV is the voltage across 2j , then

2 2

2 22 2

22 j J jj j j j

C dV dCdq di C V Vdt dt dt dt

From the above equation, we see that if

dvdt

is large, 2

1 j will be large. A high value of

charging current may damage the thyristor and the device must be protected against high

dvdt

.

The manufacturers specify the allowable

dvdt

.